The present invention relates to battery monitoring apparatus and more particularly to a battery state-of-charge indication system.
Storage batteries are used in numerous applications where it is important to know the amount of available energy remaining in the battery. For example, the battery state-of-charge is a critical parameter in the operation of battery energized electrically propelled traction vehicles, such as electric cars and forklift trucks. Such vehicles must rely upon the energy stored in the on-board batteries for propulsion, and the replenishing of stored energy requires special equipment which is only available at a charging station. Thus, means for indicating the energy state of the remaining battery charge can be advantageously used by the vehicle operator to ensure that the vehicle is returned to a charging station before the battery has been completely discharged. The vehicle batteries represent a substantial investment and the amortization of battery costs depends upon the available number of charge/discharge cycles and upon the average depth of discharge of a lead acid battery. It is well known that the life of a battery is reduced significantly when it is repeatedly discharged such that the specific gravity of the electrolyte falls below a specified quantity. Hence, it is desirable to provide some means for recognizing that the battery state-of-charge is approaching this level. Further, it is important that such a state-of-charge indicator system provide a continuous and accurate state-of-charge output while the battery is connected to its normal load circuit and is operating under normal load conditions. For example, in the case of a battery energized vehicle, this permits the operator to perform his mission until the batteries have been discharged to a desired level.
Various types of prior art systems have been proposed for indicating the energy remaining in a battery or detecting a low battery condition under normal load operation including the following.
Specific gravity metering devices can detect a change in the index of refraction with variations in the specific gravity of the electrolyte. However, loads are commonly energized by plural batteries, each battery comprises a larger number of cells and the electrolyte condition varies from cell to cell. This requires multiple sensors to obtain an average specific gravity. This results in unduly complex sensing circuits and means for interconnecting the sensors to the indicating system so as to permit normal battery exchanges. Further, the specific gravity of a cell may vary throughout the cell so that no one location is ideal as a sensing point.
Ampere hour and watt hour meter devices operate on the assumption that the remaining available energy corresponds to the energy input during charging minus the electrical energy which has been extracted. However, such systems fail to account for loss in the remaining available energy resulting from increased current discharge rates. For example, ampere hour meters commonly utilize a reversible electrochemical plating cell. During battery charging, this cell is plated with a material at a rate corresponding to the magnitude of the charging current so that total plating is the product of current time. During discharge, the plating process is reversed in a similar fashion. However, the ampere hours (AH) available from a lead-acid battery depend upon the rate of discharge. Thus, a battery rated at 300 AH at a current drain which would deplete the charge in 6 hours may only provide 220 AH at a current corresponding to a one hour discharge rate so that the meter may indicate that one-third of the energy is available when, in fact, the battery is completely discharged. In addition, the instrument must remain with the batteries when batteries are exchanged for an alternate set at the charging station, since the meter cannot be reset for the unknown charging history of the new set. Further, such devices are subject to additional inaccuracies resulting from variations of recoverable AH based on battery age and temperature and variable battery charging efficiencies.
Arrangements have been proposed for indicating state-of-charge based on sensing the terminal voltage of batteries while energizing their load circuits. Commonly, a heavily damped voltmeter is connected directly across the battery to provide an indication of battery charge. The battery voltage varies substantially with the changes of discharge current encountered during operation of loads such as the electric motors of traction vehicles. Thus, the meter produces variable and erroneous indications of battery charge. An electric vehicle operator may judge the battery charge condition by the magnitude of voltage drop during a specific maneuver such as acceleration, i.e., a specific load. This requires a high level of skill and close observation by an operator who is likely to be preoccupied with vehicular operation.
Battery condition monitors utilizing a similar voltage detection principle have been employed in some battery powered industrial trucks. These devices have a voltage level switch activated when the battery terminal voltage drops below a preset level, e.g., 80-85% of nominal voltage. If the voltage remains below this level for a preset time interval, e.g. 15-30 seconds, an indicating lamp is energized and a second timer may be started. Upon the presence of the undervoltage condition over this second time interval, a specified work function of the vehicle, such as the forklift, can thus be disabled so as to force the operator to return to the charging station. However, the sensing and detection means of such monitors are inexact and dependent upon many variables, including the changes in battery voltage with variations in load. In addition, no continuous indicating means is available to continuously advise the operator of the present state-of-charge and the low charge indication will often catch the operator by surprise.
Battery state-of-charge indicating systems relying on detection of battery terminal voltage only during periods of load, i.e., only when subject to discharge current, have tended to be inaccurate. Improved results have resulted from a battery monitoring system providing an indication of the state-of-charge based on the differences between nominal battery voltage of a fully charged battery at a predetermined discharge current and the actual battery voltage occurring at the same predetermined discharge current level. A measure of battery voltage is stored when the predetermined value of discharge current occurs such that the stored value is updated solely during the occurrence of such predetermined value. Thus, the stored value provides a continuous indication of battery state-of-charge. This battery monitoring apparatus which is disclosed in U.S. Pat. No. 4,021,718, assigned to the assignee of the subject application, utilizes discharge current measuring means to produce a signal proportional to load, or discharge current. However, if such a current measuring device is not otherwise required for control of the battery load circuit, its requirement solely for use in battery state-of-charge monitoring increases the cost of the battery monitoring system which may limit its use in a highly competitive market.
An improved battery state-of-charge indicator is disclosed in our co-pending U.S. patent application Ser. No. 29,941 filed Apr. 13, 1979, now U.S. Pat. No. 4,234,840, and assigned to the assignee of the present invention. In that application we disclose a system for monitoring battery voltage and for storing a scaled value representative of battery terminal voltage. The stored value of terminal voltage is decreased at a predetermined rate which is independent of actual battery discharge current. When the scaled value of actual battery terminal voltage exceeds the stored value, the stored value is increased to the scaled value at a rate substantially faster than the predetermined discharge rate such that the stored value is indicative of battery voltage under no-load conditions and represents the battery state of charge. The stored value is decreased at a predetermined low rate when the battery is under load and is decreased at a faster rate under no load conditions. The faster rate can be varied as a function of the difference between the scaled value of actual battery voltage and the stored value.
Although this last described system provides an adequate representation of battery state-of-charge when the battery experiences frequent intervals of no-load conditions, this system is not totally accurate when the battery is subject to long periods of variable current drain with infrequent no-load conditions.